Shifting The Powerband

[gt-ahhh!]

hey all,

ive got an r33 rb25det with all the usual mods for 250rwkw.

the turbo is a gcg hi-flow. it chucks out unusually high kw in the top end, and keeps making power well past the redline, but unfortunately this is at the expense of midrange, which is down approx 20-30kw compared to similar setups. my tuner and i believe the rb25 head i installed must have a set of camshafts for the dyno curve to behave this way. (see attachment)

this is all fine in a straight line, but on the street it can be very sluggish below 70kmh (unless you try to crash it into 1st i guess lol)

my question is this: would an adjustable ex. cam gear improve this? to what extent? or would i be better off going back to stock/other camshafts?

i believe the vct is functioning, and that choosing a cam gear is probably cheaper in terms of labour and tuning?

thanks!

[PM-R33]

Take a cam cover off and check to see what cams are inside if you think they are not stock.

Edited May 6, 2010 by PM-R33

[gt-ahhh!]

i don't think they had obvious markings on them mate.

[R31Nismoid]

3500rpm - 16psi

6000rpm - 18psi

Problem

IMO if you fixed the 2psi you are missing that could just be all you need to make it feel better.

It'll also improve the entire car as it takes 2500rpm to slowly increase to your peak for whatever reason.

It probably won't give you an extra 30kw or so where you feel you are missing it, but it will give you a bit more punch and drag the entire graph to the left a bit more.

ie.

180rwkw @ 4900rpm as it

or

180rwkw @ 4500rpm (potentially) with the extra boost in there earlier

Failing that, get the cams out and measure them and go from there.

[gt-ahhh!]

it has an electronic boost controller... so i wonder if fiddling with that could help?

3500rpm - 16psi

6000rpm - 18psi

Problem

[jarrod83]

maybe also check the cam positions with respect to the crank.. i.e that the harmonic balancer and cam gear marks all line up.. if the cams are advanced you get more bottom end, more top end if they are retarded.. at a guess id say with a older timing belt it may be possible for it to stretch slightly and retard the cams.. make sure you know how to set the base timing back up before you attempt this

don't quote me on it tho its just conclusions i have arrived at while trying to work out why i my power curve dips off so aggressively in the top end while i have mountains of bottom end torque..

an adjustable exhaust cam gear wouldnt hurt to try and find a better compromise in performance, but it all comes down to dyno time .. are you sure your vct is working? and if they are aftermarket and still have the vct they would most likely be poncams?

also when is your boost coming on?? is it laggier compared to other setups?

Hope that helps

Jarrod

[R31Nismoid]

it has an electronic boost controller... so i wonder if fiddling with that could help?

Well something is not quite right with the boost control either way mate so it needs attention.

I'm not saying it's going to make the car feel 100% better.

There could be other things as others have suggested - around the cam timing, ensuring VCT is actually working etc etc.

But either way that is something that needs attention as it slowly climbs over 2500rpm suggesting the boost controller is not quite doing a perfect job.

Could be the install/settings or just need something better.

[paulr33]

as the others have said check basics

check your VCT is working

check base timing at idle

how is it tuned? is there enough timing in the tune (does it ever knock?)

how do the AFR's look ?

why do you have a reason to suspect it has different cams?

my rb25det is bog stock, i have no reason to :guess: it has different cams, why do you?

has something else changed or been done to the engine that you havent posted?

[gt-ahhh!]

maybe also check the cam positions with respect to the crank.. i.e that the harmonic balancer and cam gear marks all line up.. if the cams are advanced you get more bottom end, more top end if they are retarded.. at a guess id say with a older timing belt it may be possible for it to stretch slightly and retard the cams.. make sure you know how to set the base timing back up before you attempt this

don't quote me on it tho its just conclusions i have arrived at while trying to work out why i my power curve dips off so aggressively in the top end while i have mountains of bottom end torque..

an adjustable exhaust cam gear wouldnt hurt to try and find a better compromise in performance, but it all comes down to dyno time .. are you sure your vct is working? and if they are aftermarket and still have the vct they would most likely be poncams?

also when is your boost coming on?? is it laggier compared to other setups?

Hope that helps

Jarrod

all good points. the vct is def working. i know this because the electrical socket broke not long ago, and it was VERY noticeable!

boost, 16psi @ 3500rpm. so thats pretty good id reckon! its a smooth curve between 16-18psi across the range.

i dont know that much about cams, but yes my tuner did describe it as a poncam.

[jarrod83]

mitch are all 3 dyno graphs yours or just the red one?

[gt-ahhh!]

as the others have said check basics

check your VCT is working

check base timing at idle

how is it tuned? is there enough timing in the tune (does it ever knock?)

how do the AFR's look ?

why do you have a reason to suspect it has different cams?

my rb25det is bog stock, i have no reason to :guess: it has different cams, why do you?

has something else changed or been done to the engine that you havent posted?

all these things were checked, or should have been, as it was professionally tuned. as for afr's, i havent actually seen them myself, so i cant comment. but that might be worth a look.

the main reason i suspect, and have been advised, the cams are non-standard is because of the top end. generally that turbo wont make much over 240rwkw peak power. and since its designed to be close to stock response, it shouldnt be making more and more power past redline like mine does lol.

before i made this topic i looked up dyno curves of similar setups and they were all making more midrange but less top end than me!

i cant think of anything usual done to the motor, its a stock head gasket... so normal comp.

other mods - injectors, ebc, whole exhaust, intake manifold, fmic, bov, map sensor ecu, splitfires... all the usual stuff.

[gt-ahhh!]

mitch are all 3 dyno graphs yours or just the red one?

all mine. the green run was the best, but the timing was wound back a touch to be street safe, which is the blue run.

the red run, and red boost curve is for the low boost mode on my ebc.

[R31Nismoid]

all good points. the vct is def working. i know this because the electrical socket broke not long ago, and it was VERY noticeable!

boost, 16psi @ 3500rpm. so thats pretty good id reckon! its a smooth curve between 16-18psi across the range.

i dont know that much about cams, but yes my tuner did describe it as a poncam.

It should hit 18psi, not take another 2500rpm to slowly climb up there.

the main reason i suspect, and have been advised, the cams are non-standard is because of the top end. generally that turbo wont make much over 240rwkw peak power. and since its designed to be close to stock response, it shouldnt be making more and more power past redline like mine does lol.

Not sure where you got that from but the R33 GCG hi-flow will make around 250rwkw (basically what you have).

The R34 GCG Hi-flow gets to around 260-270rwkw due to the larger rear etc.

It seems like the response is about right, just need to fix the minor niggles.

Have you dropped the exhaust (just outta interest), to see if it's perhaps posing restriction which might deaden it a bit as well.

Sometimes a car will still make power even with a restriction if it's not overly bad.

I think you just need to look @ all things suggested thus far, and THEN see.

Can't offer much more advice than what is here now really as you've now got a few things to go check out

[SKYPER]

To be 100% sure they are aftermarket cams, pull camcovers off and measure the height of lobe and base circle diameter with vernier clamp.

Aftermarket cams should increase mid to high hp, torque. Can create a bow look to dynograph at lower rpms when cams have enough duration, lift. Also aggressive enough cams should create a lumpy idle (or stock / Tomei cams that had camgears adjusted). Tomei Poncams probably would have a smooth idle and look similar to stock cams, unless you adjust the camgears to make idle lump.

This dynograph shows what happens when intake, exhaust cams are adjusted on RB20DET (should be similar with RB25DET) -

http://www.uniqueautosports.com/Dyno/yx32gtstdyno.gif

But VVT plays a role with powerband, so I would be looking at VVT settings.

Tomei has a section on their website ("Making a good use of VVT") that talks about how VVT affects powerband -

http://www.tomei-p.co.jp/_2003web-catalogu...8_camshaft.html

The first example under the heading (left side) where midrange is lost looks similar to your dynograph.

Edited May 6, 2010 by SKYPER

[gt-ahhh!]

Not sure where you got that from but the R33 GCG hi-flow will make around 250rwkw (basically what you have).

The R34 GCG Hi-flow gets to around 260-270rwkw due to the larger rear etc.

It seems like the response is about right, just need to fix the minor niggles.

Have you dropped the exhaust (just outta interest), to see if it's perhaps posing restriction which might deaden it a bit as well.

Sometimes a car will still make power even with a restriction if it's not overly bad.

I think you just need to look @ all things suggested thus far, and THEN see.

Can't offer much more advice than what is here now really as you've now got a few things to go check out

haha ash you really know everything dont you! so you reckon i havent got extra top end at all?

im just basing the 240rwkw figure on the dyno graphs ive seen. my main concern is with the nature of the power curve, being too top heavy. i will def have to go talk to the tuner about the points everyone has raised.

in regards to the exhaust, its got a 3in split dump. a 3in cat that had an unfortunate incident with a hammer and screwdriver. behind that, i had the centre muffler cut out and its all 3.5in jasma straight through resonator and muffler. i did take it off once for a laugh, but i dont imagine it would be too restrictive haha

can resonators get blocked up? i know the mufflers not, i can see through it!

[R31Nismoid]

Well you do have a tad higher top end - but the GCG hi-flow you have would be just about on its limits really, i think the highest reading posted over the years is around 275rwkw (r34 item) and that was the absolute limit. Anymore boost just didn't deliver anything useful.

Have you punched 20psi into it just to see? (obviously back it off after a couple runs).

Also - did you pipe the CAT? Just hollowing out a CAT can cause undue turbulence - better to jam a pipe down the middle.

So there is something else you have to do... With the list of small things, it will add upto perhaps a nice end result.

But definately get the cams checked as well.

[gt-ahhh!]

thanks SKYPER

interesting reading. the cam gear adjustment graph looks promising.

[femno]

My bet is that its a OP6. Thats where your midrange went. But its hardly laggy?????

33 Hiflows dont keep making that much power up top. Especially not on 18psi. Its too much heat for those tiny housings.

On the other hand OP6s do continue to pull big numbers all the way to redline with a little midrange sacrifice.

Chuck on an adjustable EX gear and it will help. Maybe fiddle with boost control but its not that far off.

[gt-ahhh!]

Have you punched 20psi into it just to see? (obviously back it off after a couple runs).

Also - did you pipe the CAT? Just hollowing out a CAT can cause undue turbulence - better to jam a pipe down the middle.

So there is something else you have to do... With the list of small things, it will add upto perhaps a nice end result.

But definately get the cams checked as well.

yeh its pretty close to its limit. they wound more boost in but it wasnt making any more power so no point. the dyno graph actually shows it makes the same peak power at 15psi, but less across the range obviously.

no the cats not piped. my logic being if i ever get done for it ill just say it broke. not sure if theyd ever pull it off and notice a pipe though...

[gt-ahhh!]

My bet is that its a OP6. Thats where your midrange went. But its hardly laggy?????

33 Hiflows dont keep making that much power up top. Especially not on 18psi. Its too much heat for those tiny housings.

On the other hand OP6s do continue to pull big numbers all the way to redline with a little midrange sacrifice.

Chuck on an adjustable EX gear and it will help. Maybe fiddle with boost control but its not that far off.

what is an op6?

yeah i reckon the turbo comes on pretty well, its just that the pull is gradual when it does!